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Howlett" , Vlastimil Babka , Mike Rapoport , "Suren Baghdasaryan" , Michal Hocko , "Balbir Singh" , , , , , , , Subject: Re: [PATCH v5 1/5] mm/zone_device: Reinitialize large zone device private folios Message-ID: References: <20260114192111.1267147-1-francois.dugast@intel.com> <20260114192111.1267147-2-francois.dugast@intel.com> <20260114134825.8bf1cb3e897c8e41c73dc8ae@linux-foundation.org> Content-Type: text/plain; charset="utf-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-ClientProxiedBy: BY3PR05CA0037.namprd05.prod.outlook.com (2603:10b6:a03:39b::12) To PH7PR11MB6522.namprd11.prod.outlook.com (2603:10b6:510:212::12) X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH7PR11MB6522:EE_|SA2PR11MB4796:EE_ X-MS-Office365-Filtering-Correlation-Id: 82b5003e-13df-436e-8938-08de53c7d069 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|376014|1800799024|7416014; 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This step is necessary when the folio’s order changes > > > dynamically between zone_device_page_init calls to avoid building a > > > corrupt folio. As part of the metadata reinitialization, the dev_pagemap > > > must be passed in from the caller because the pgmap stored in the folio > > > page may have been overwritten with a compound head. > > > > Thanks. What are the worst-case userspace-visible effects of the bug? > > If you reallocate a subset of pages from what was originally a large > device folio, the pgmap mapping becomes invalid because it was > overwritten by the compound head, and this can crash the kernel. > > Alternatively, consider the case where the original folio had an order > of 9 and _nr_pages was set. If you then reallocate the folio plus one as s/_nr_pages/the order was encoded the page flags. Not clearing _nr_pages is probably bad too, not sure what the side affect of that is, but it can't be good. > an individual page, the flags would still have PG_locked set, causing a > hang the next time you try to lock the page. > > This is pretty bad if drivers implement a buddy allocator for device > pages (Xe does; Nouveau doesn’t, which is why they haven’t hit this > issue). Only Nouveau enables large device pages in 6.19 but probably > best to have kernel flying around with known issues. s/best to have kernel/best to not have kernels Matt > > Matt